Development of better bulk growth techniques is an urgent requirement in meeting the demand for better crystal materials with direct band gaps, higher electron mobilities and less susceptibility to radiation damage. In particular, compound systems such as III-V and II-V and various alloy systems offer attractive properties for a variety of applications requiring such characteristics.
The growth of high quality single crystalline Si in bulk quantities has been possible for a number of years. This, however, is not the case for the III-V and II-VI compound or alloy-type systems. Growth in these systems is complicated by the presence of more than one component which is often volatile and whose composition must be precisely controlled. Alloy-type systems are even more difficult because of macrosegregation resulting from the large segregation coefficients, density differences between constituents, and buoyancy-driven convective flows driven by thermal or solutal gradients. Such flows can redistribute solute, transport contaminants from the crucible, and produce fluctuations in the growth environment which can lead to various defect formation. Dislocations can be produced by unfavorable interface shape (i.e., concave to the melt), thermal strains, and mechanical stress from confining vessels. The most widely used techniques for bulk growth of compound semiconductors have been horizontal Bridgman using an open load in a pressurized vessel, liquid encapsulated Czochralski, and vertical Bridgman-Stockbarger in a closed ampoule. For GaAs, the horizontal Bridgman technique generally produces material with fewer dislocations, presumably because of the open boat design which allows the material some freedom to expand on freezing. However, if a dopant is added, the uncontrolled convection inherent in the process causes the dopant concentration to vary continuously along the length of the sample.
Single crystalline Si has been effectively grown in bulk by means of float-zone processes. In this type of process a polycrystalline rod supported at each end is passed through a heated region so as to form a molten zone that progresses along the rod. The process is containerless and it depends on surface tension and Lorenz forces from the induction heating coil for support of the molten zone.
Application of float-zone processes to compound and alloy systems, however, is beset with difficulties owing to differences between the properties of Si and these materials. Limitations to extending Si float-zone growth to other materials are as follows:
1. Si has an unusually large surface tension (ten times that of H.sub.2 O) which helps support large molten zones. 2. Molten Si is a good electrical conductor which allows RF induction heating which also provides additional Lorentz forces to help support the molten zone. 3. Even though it is possible to eliminate dislocations in float zone Si, there are severe growth rate fluctuations and thus compositional striations caused by the heating asymmetry inherent in the RF work coil and by uncontrolled convection driven by buoyant as well as surface tension forces. In fact a recent space experiment indicated that surface tension-driven convection may be the dominant cause of such striations. The growth rate fluctuations are not particularly serious in an intrinsic elemental semiconductor such as Si, but could produce serious inhomogeneities and defects in extrinsic, alloy type, or compound semiconductors. Another difficulty with conventional float-zone growth of multicomponent systems is the control of stoichiometry since the the more volatile component will tend to evaporate at the free surface. Liquid phase encapsulants cannot generally be used because of gravity drainage. Maintaining an over-pressure of the volatile component may not be possible because of the temperature variations associated with the process since the maximum pressure of a particular component that can be obtained is the vapor pressure of the component in question at the temperature of the coldest region in the pressure vessel. Finally, the float zone process cannot be successfully applied to many materials on Earth because of their low surface tension. This is especially true for poor conductors because Lorentz forces cannot be used to help support the molten zone.
It is therefore an object of this invention to provide a float-zone process for growing crystals of compound and alloy systems in which dislocations in the grown crystal are significantly reduced or eliminated.
Another object is to provide a float-zone process wherein stoichiometry is maintained and loss of volatile components from compound systems is prevented.
Yet another object is to provide a float-zone process wherein buoyant support for the molten zone is provided.
Another object is to provide a float-zone process wherein materials having a low surface tension may be accommodated.
Another object is to provide a float-zone process for use in the microgravity environment of space.
A further object is to provide apparatus for carrying out a process meeting the above objects.